Conventionally, a pressure sensor is attached in the door of a vehicle mainly to detect a collision with the side surface of the vehicle (hereinafter referred to as a side collision) thereby to detect a pressure change in the door. When a side collision occurs with the vehicle, the volume of the inside of the door changes, and its pressure change is detected by the pressure sensor attached. Consequently, the side collision with the vehicle is detected, and an occupant protection device such as an airbag device or a seat belt pretensioner is actuated to protect an occupant. The detection result by the above pressure sensor serves as a trigger for actuating the occupant protection device, and thus it should be an accurately-detected pressure change in the door. Thus, when the pressure sensor has failed, it has been necessary to detect this failure with precision, thus preventing the erroneous operation of the occupant protection device.
Regarding this issue, there is a conventional art relating to the method of detecting the failure of pressure sensors whereby to compare the detection values from a pair of pressure sensors provided in the right and left doors of a vehicle with each other and to determine that either one of the pressure sensors has failed when both the detection values are different beyond a predetermined degree (see, for example, Patent Document 1). In view of extremely rare failures of both the pressure sensors at the same time, the failure of either one of the pressure sensors can be detected by this conventional art. When the vehicle is located at high altitudes, both the pressure sensors detect abnormal values due to the change of the surrounding atmospheric pressure. Thus, the above conventional art can detect whether either one of the pressure sensors has failed or not separately from the abnormality of the pressure sensor due to the elevation where the vehicle is located.